Beating the dome

The domed ceiling of the new Queen Alia International Airport terminal in Jordan created a challenging echo for the audio teams to tackle. James Ling finds out more.

In the Middle East, domed ceilings are a common feature. They have been used in mosques for hundreds of years and form a distinctive part of the architectural heritage of the region. However, the domed ceiling built for the new terminal at Queen Alia International Airport in Jordan is not like any other structure in the region.

‘Domes in mosques are typically much smaller if you look at the radius of the dome itself. The radius of a dome in a mosque, even a large one like the Grand Mosque in Abu Dhabi is maybe 25-30m, so the focus point of those domes will be somewhere up in the roof,’ explains Jakob Kraft from Rahe-Kraft Acoustic Consultants. ‘What you will not have in a mosque is a focused reflection that people will hear.

‘In the case of Queen Alia airport, the radius is not like a half circle, it’s a very shallow shell,’ continues Mr Kraft. ‘If you stand right underneath one of the domes you have a flutter echo that moves to the outside of the dome and then comes back to you. You have an effect that you wouldn’t experience in a mosque because those areas are not accessible. In Queen Alia, those areas are under every dome, right where passengers are moving.’

The initial design of the airport had the domes made entirely from raw concrete. However, the contractor was alerted to the problem the flutter echo would cause and acoustically treated them. Despite this action, the domes were still the major challenge in designing an intelligible public address system.

This was where Rahe-Kraft came in, as it was contracted to design a loudspeaker system suitable for the space. ‘The most important decision we made was we couldn’t place a loudspeaker anywhere in the centre of those domes because it would still lead to the same kind of flutter echoes even if you used very directive loudspeakers,’ explains Mr Kraft.

The solution suggested by the consultancy was based around DSX Series loudspeakers from Duran Audio. Reasons for this selection were based on its previous experience using the manufacturer’s equipment and its ease of programming. But there were also factors specific to this project which made Duran Audio the appropriate choice.

‘There were a couple of cases where we needed very wide horizontal directivity,’ furthers Mr Kraft. ‘In the piers, they are very stretched long buildings and there we needed very wide horizontal speakers. Typically if you use just a 4-inch cone speaker the directivity narrows down above 4-5kHz. That is why we used the DSX series, which has dedicated high frequency drivers with a small horn attached to it. They are much wider in horizontal directivity and that was necessary in those areas.’

A further reason was due to the needs of the architect, who did not want any equipment installed on the structural columns in the airport, favouring the raw concrete appearance. With this stipulation, the design was based around a series of poles placed at strategic locations in the airport with sometimes multiple speakers attached to them.

‘The resulting directivity of something like that is difficult to control and to calculate because you have a lot of interference patterns,’ explains Mr Kraft. ‘With Duran Audio it was possible for me to create a directivity that was valid for the whole pole. This was necessary as you would have no control if you couldn’t treat the whole totem as one unit.’

The consultant used Catt Acoustic software to design a solution that would cover the space with the desired intelligibility and STI. Over 120 Duran Audio speakers were specified throughout the airport to form its line array solution. Added to this is a Bosch Praesideo system consisting of 110 cabinet and nearly 1,400 ceiling speakers for the public address solution.

Installing the airport

With the system designed, responsibility for the installation fell on systems integrator Energy International. The company ensured it was thoroughly prepared for the project, with its staff being sent for refresher courses on Praesideo as well as Duran Audio training as it wanted its engineers to ‘have the knowhow before it got to crunch time’.

As the installation team worked through the project, the domes once again began causing concern due to echoes being heard during Management walks for progress reviews and as the building neared completion. ‘We had a plan to bring in Rahe-Kraft when everything was ready for final testing and configuration so they could complete the entire airport in one shot,’ recalls Mike Shararah VP Controls for Energy International. ‘However, we quickly realised that if we continued with this plan then what if the actual results are not as was predicted theoretically? What if we get a lot of echoes and low intelligibility? From an execution perspective, we will be in trouble since there will not be ample time for a plan B,’ he explains. ‘So we decided to make sure we got it right and invited Jakob to work on the North Pier and as much as we can of the South Pier first, to get at least some idea of what results we would get at this airport leaving enough time for an alternate course of actions if needed.’

The team from Energy International, led by ELV manager Hasan Sourjah, worked to complete the airport’s new North Pier and get as much done as possible on the South Pier (which mirrors the other pier’s design) prior to Mr Kraft arriving in Jordan.

‘We brought in experts in PA systems from Sri Lanka, two guys who are experienced at testing and commissioning to catch the installation problems we knew we had,’ explains Mr Shararah. ‘They went around and made sure that everything was ready, properly installed, and connected 100 per cent correctly. In addition, the polarity of speakers was checked prior to Jakob arriving. We wanted him to be able to get straight on with the testing, rather than having to fix issues, which he normally faces on other airport projects.’

This approach clearly worked. ‘I was very happy with the way that the system was prepared,’ confirms Mr Kraft. ‘It is a rare experience anywhere in the world that you make a list of requirements that need to be complete when you arrive on site and then you find the system in that condition. That has never happened to me except in Queen Alia Airport thanks to Hasan. He is really the best project manager I have met both in terms of practical knowledge but also in terms of motivating his crew.’

‘Jakob was able to finish the North Pier and most of the South in a short period,’ continues Mr Shararah as he explains to Pro Audio Middle East the testing phase. ‘We had the acoustic results of Jakob’s first trip and we realised immediately that we were going to deliver an airport on time and deliver a high-quality system despite the challenges,’ he smiles. ‘After that, we prepared the main building. Jakob came again and it was the same story there – it was relatively quick, no hassles and no issues.’

Good communication and teamwork between the contractor, consultant and the systems integrator has ensured that while there are still challenges inside the airport, you do not get any flutter echo from the PA. This becomes apparent when you move outside the building into the untreated areas. ‘If you walk outside the main doors of the airport you have the same dome structure outside,’ says Mr Shararah.

‘There is no acoustic insulation in those areas, and if you stand under the centre of a dome and speak you not only hear your echo, the sound travels to the adjacent domes. You can hear a conversation that somebody else is having a good distance away.

‘Imagine if the contractor didn’t have Jakob early on to recommend acoustic insulation,’ he cautions. ‘Imagine if we didn’t have the proper system in which he recommended the placement of the arrays as much as possible on the edges of the domes and away from the centres.’

The end result is one that everybody concerned takes pride in. ‘Overall the resulting intelligibility was quite good. And the subjective intelligibility is actually much better for a line array system than it would be for a distributed system like a ceiling system with the same measured STI,’ reasons Mr Kraft.

‘I think those dome shaped roof segments really make this building special in terms of acoustic properties,’ he continues. ‘It’s not the kind of building where you can claim that a prediction will be 100 per cent correct, so we do think that this is pushing prediction software as it is pretty much to the limits. At the same time I have to say that we were very happy using Catt Acoustics.’

‘It went smoothly for the airport because we had experts early on,’ concludes Mr Shararah. ‘Jakob came in early on and did an acoustic study. He recommended certain things and he knew where the risks were. The contractor ensured that the acoustic insulation went up in the domes. Our engineers selected the right equipment and paid attention to detail. Collectively, the efforts paid off to have the best acoustic results possible in the space with today’s technology.’

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